1. Field of the Invention
The present invention relates to a dual range scale for weighing and counting of articles, and more particularly to dual range load cells utilizing two tandem connected load cells in which one cell covers a low range of weights and a second cell covers a high range of weights.
2. Background of the Invention
In weighing and counting scales, it is common to have a requirement for high accuracy over a very wide range of weights. For example, in inventorying parts, there may be a large number of light weight units, but having a large gross weight, followed thereafter by a relatively small number of lightweight objects in which resolution must be made at the lower end of the scale. Thus, there is a requirement for a counting type scale which will provide very high accuracy for light weights and low counts as well as for high counts and heavy weights. It is common, in scales of this type, to utilize load cells such that processing of the signals produced can be done electronically. A low range load cell for use with lighter weights may be used in one instance, and a high range load cell for a heavier weight in another instance.
It has also been known to join individual cells in tandem. For example, in U.S. Pat. No. 3,120,754 to Lebow, two load cells are shown in tandem and arranged with mechanical stops to permit overloading of the low range cell without damage when the weight exceeds its range, reload is then operative on a second, higher range load cell. This disclosure teaches cells arranged end to end for use in tension. U.S. Pat. No. 3,464,508 to Engle et. al., illustrates the use of multiple cells; however, the cells are basically in parallel for measuring a distributed load by summing the outputs from each cell. It is also desirable in multiple cell type scales for the load capacity switching to be done automatically without the attention of the operator. One such electrical scale changing system is shown in the U.S. Pat. No. 3,437,160 to Hill, which utilizes a complex servo system for that purpose.
There is a need for a wide range scale system which will have high sensitivity at low weights or counts, which will be of a compact design, and in which switching of scales is performed electronically without attention of the operator.